An index is a rule somebody wrote, not a fact about the market

The points below about how indices are built are ordered by how much difference they make, not by how often they get repeated.

Somebody chose the constituents

An index is a published rule set: which securities qualify, how many are included and how often the list is reviewed. Those rules are written by a commercial index provider, and different providers make different choices about the same market.

Free-float adjustments, minimum liquidity requirements and domicile rules all change what ends up in the list. The result feels like a neutral measurement of the market and is a specific construction of it.

Weighting is the largest decision

Most widely used indices weight by market capitalisation, so the largest companies occupy the largest share. That means holding a market-cap tracker gives you more of whatever has already risen and less of what has fallen, automatically.

In numbers, defenders point out this requires no trading and reflects how capital is actually allocated; critics point out it concentrates exposure in whatever is currently expensive. Equal weighting, revenue weighting and other schemes exist and produce materially different behaviour and different costs.

Concentration is a live consequence

When a small number of very large companies dominate a market, a cap-weighted index of that market becomes highly concentrated in them. This has happened repeatedly in different markets and eras, and the level of concentration varies over time.

On the balance sheet, an investor who believes they are diversified across hundreds of companies may have a large share of their money in a handful. The fund's published top-ten holdings and their combined percentage is the number to look at.

Rebalancing has a cost

When constituents change, tracking funds must buy and sell, and those trades happen on known dates that other market participants can anticipate. The associated costs are borne by the fund and show up as tracking difference rather than as a headline fee. Indices with frequent turnover impose more of this cost than stable ones, which is one reason niche indices can be more expensive than they look.

Comparing a fund's actual return to the index it tracks over several years reveals this better than any published figure.

Tracking method changes what you own

Physical replication means the fund holds the underlying securities, either all of them or a representative sample. Synthetic replication uses derivatives to deliver the index return, which introduces exposure to the counterparty providing them. Both are legitimate and widely used, and the choice affects the risks, costs and sometimes the tax treatment.

For most households, the fund documentation states which is used, and it is worth knowing which one you own.

The right answer depends on your tax situation, which this cannot see.

What indexing does and does not promise

It promises the market's return minus costs, which historically has been enough to beat the majority of active attempts after fees. It does not promise a positive return, protection in a fall, or that the index represents a sensible portfolio for you.

A market can be flat or negative for extended periods, and tracking it faithfully means tracking that too. Whether index funds suit your circumstances is a question for regulated advice rather than a general conclusion.

Everything above, in order of what to do first

1. Somebody chose the constituents. An index is a published rule set: which securities qualify, how many are included and how often the list is reviewed.
2. Weighting is the largest decision. Most widely used indices weight by market capitalisation, so the largest companies occupy the largest share.
3. Concentration is a live consequence. When a small number of very large companies dominate a market, a cap-weighted index of that market becomes highly concentrated in them.
4. Rebalancing has a cost. When constituents change, tracking funds must buy and sell, and those trades happen on known dates that other market participants can anticipate.
5. Tracking method changes what you own. Physical replication means the fund holds the underlying securities, either all of them or a representative sample.
6. What indexing does and does not promise. It promises the market's return minus costs, which historically has been enough to beat the majority of active attempts after fees.

The takeaway

Read the index rules and the top ten holdings. Neutral is a description of the method, not of the result.

Costs compound as reliably as returns do, and in the same direction.